grdvector and PDF format: gigantic files

**Description of the problem**

I have a fairly complex script (modern style) that plot a single map (using Lambert projection) on which displacements vector are plotted using grdvector.
When outputting to PS or EPS, the file is about 1.7 MB.
When outputting to PDF, the file is over > 700 MB.

- The same issue arise when trying to convert the PS/EPS files to PDF using other software.
- The PS/EPS can be visualized without any issue with Evince (document viewer on Ubuntu).
- No issues when outputting to PNG in GMT.
- No issues when outputting to PDF when removing the `gmt grdvector` command line from the GMT script.

**Actual outcome**

A PDF file 400 times bigger than the PS/EPS formats.

**Expected outcome**

A PDF file with a size similar to the PS/EPS formats.

**System information**

* Operating system: Debian/Centos
* GMT version (`gmt --version`): 6.2.0